The present invention relates to a mobile telephone battery power supply unit which includes a power detection circuit controlled to detect the current power level of the rechargeable battery of the mobile telephone, and a discharge circuit controlled to discharge the residual voltage out of the rechargeable battery of the mobile telephone before each recharging operation so as to eliminate possible "memory effect". The mobile telephone battery power supply unit comprises a battery case fastened inside the housing thereof and covered by a slide cover, which battery case has batteries that keep providing a constant voltage to the mobile telephone for normal operation as the rechargeable battery of the mobile telephone fails.
A variety of mobile telephones are known and widely in use for the advantage of mobility. However, a mobile telephone must be frequently recharged so that the rechargeable battery of the mobile telephone can be constantly maintained at high level for normal operation. Because of the lack of a power detection means for automatically detecting the current power level of the rechargeable battery of the conventional mobile telephone, the user may forget to recharge the rechargeable battery before it turns to low level. Furthermore, the rechargeable battery of a conventional mobile telephone is generally a nickel-cadmium cell which tends to produce a "memory effect", more particularly after a long use, causing the rechargeable battery not to be fully recharged to the saturation state.